In general, two transmission models can be used in an RFID system between a reader and a tag or transponder. The first model is a backscattering model, which is similar to a radar system, where the tag or transponder reflects the electromagnetic waves sent by the reader. The backscattering model is used mainly at higher frequency ranges, for example 915 MHz, 2.45 GHz and beyond. The antenna used in the backscattering model is a far-field antenna, for example dipole, slot or patch antenna. The planar area of an antenna used in the backscattering model is normally larger than at least 0.25 wavelength, for example about several centimeters for 2.45 GHz frequency. The second model is a magnetic coupling model and involves magnetic coupling between the reader and the tag or transponder through a pair of coils, which are the antenna for the reader, on the one hand, and the antenna of the tag or transponder, on the other hand. The second model is usually used for RFID applications with a short detecting distance at lower frequency range, for example 13.56 MHz.
There are two methods to build an antenna for an RFID tag. The first method is to fabricate the antenna on an assembly substrate and then assemble the antenna with a chip to form the tag or transponder. Antennas of this kind can be termed “off-chip” antennas as the antennas are not fabricated directly on the chip. The second method is to fabricate the antenna directly on the chip without additional assembling process, which is called an on-chip antenna (OCA). The OCA has a few advantages over the “off-chip” antenna. For an RFID tag with an OCA, the entire tag volume can be almost as small as the size of the chip of the tag. With such small dimensions, the tag can be embedded in many objects for a wide variety of applications. In addition, a low cost can be obtained for a tag with an OCA, since there are no separate assembling costs for the antenna and the chip.
The dimensions of a chip in a tag can be very small, for example in millimeter scale. Sub-millimeter dimensions are also possible if the chip is fabricated with advanced CMOS technology. In addition, the chip size can be further reduced with an increase of the tag's working frequency to a high frequency range. The reduction of the area of the tag reduces the cost of fabrication per tag.
Therefore, the dimensions of an OCA are limited by the chip's dimensions, in millimeter scale. With the small dimensions, it is not possible to fabricate a backscattering transmission model OCA on the chip of the tag for an RFID system working in a frequency range below 10 GHz. Therefore, the OCA has to use the magnetic coupling transmission model, where the antenna is a metal coil and forms a transformer with the metal coil of the reader antenna.
Several attempts have been made to fabricate an OCA on a chip of an RFID tag. Publication “A New Contactless Smart Card IC Using an On-Chip Antenna and an Asynchronous Microcontroller”, A. Abrial et al, IEEE Journal of Solid-State Circuits, vol. 36, No. 7, 2001, pp. 1101-1107 discloses an OCA fabricated on a contactless smart card chip with 0.25 μm CMOS technology. The OCA has an area of 4×4 mm2 and is working at 13.56 MHz.
Article “Maxwell Announces Coil-on-Chip RFID Technology for Life Sciences Market”, Hitachi Maxwell, discloses another RFID tag with OCA working at 13.56 MHz on a chip area of 2.3×2.3 mm2. The OCA is fabricated with 0.8-μm CMOS technology. Publication “The World's smallest RFID μ-chip, bringing about new business and lifestyles”, Ryo Imura et al, 2004 Symposium on VLSI Circuits, Digest of Technical Papers” also discloses a μ-chip with OCA working at 2.45 GHz.
U.S. Pat. No. 6,646,328 discloses an antenna structure which is fabricated on a chip. The antenna structure includes a shielding layer, a dielectric layer and an antenna layer. The shielding layer is electrically floating and includes a plurality of mutually isolated regions. The entire antenna structure may be fabricated using conventional CMOS processes and the antenna is of strip-type.
Publication “On-Chip Spiral Inductors with Patterned Ground Shields for Si-Based RF IC's,” C. Patrick Yue et al, IEEE Journal of Solid-State Circuits, Vol. 33, No. 5, May 1998 discloses a patterned ground shield inserted between an on-chip spiral inductor coil and a silicon (Si) substrate. The spiral inductor coil is fabricated within the interconnect layers in a Si CMOS process, normally with the top metal layer of the interconnect layers. The magnetic field generated by the inductor coil penetrates through to the Si substrate underneath. As the Si substrate used in a standard low cost CMOS process has low resistivity, an eddy-current is induced within the Si substrate by the magnetic field. This consumes the energy of inductor coil and lowers the inductor coil's quality-factor and performance. To prevent the generated magnetic field from penetrating into or through to the Si substrate, a polysilicon layer is arranged between the inductor coil and the Si substrate to form a shielding layer for the inductor coil.
There are discrepancies between the simulation and measurement results in the fabricated OCA structures of prior art devices. These discrepancies prove that the known OCA structures cannot be pre-designed. Therefore, an objective of the present invention is to provide an alternative OCA that advantageously avoids or at least reduces some of the above-mentioned drawbacks of prior art devices in an easy and economical manner.